The present invention relates to a process for producing polyolefins which have a very low content of impurities. The obtained polyolefins particularly have a low content of volatile low molecular weight compounds, a low content of low molecular weight polyolefin waxes, and a low content of residues derived from the catalyst employed in the polymerization.
Generally, polyolefins contain impurities as a result of the polymerization process. Typical impurities include residues from the catalyst employed during the polymerization, including the cocatalyst(s) and scavenger(s), oligomers, by-products and various volatile low molecular weight compounds. Depending on the application, the impurities may have a negative effect even if they are contained in the polymer only in low concentrations. For example, polymers used for water pipes should not contain low molecular weight volatile compounds since these compounds might migrate from the polymer into the transported water and cause taste or odor problems. Further, polymers used for making films to be used in electric devices, such as insulation films in capacitors, should not contain metal impurities in order to avoid an electrical breakthrough through the film. Finally, polymers used in automotive applications, in particular polypropylene used for the manufacture interior parts of an automobile, need to satisfy high standards with respect to volatile compounds contained in the polymer. High amounts of hydrocarbons contained in a polymer employed for preparing such interior parts may lead to fogging problems, in particular when such hydrocarbons, typically hydrocarbons having from 16 to 32 carbon atoms, evaporate from the interior parts and condense on the car windows. At the same time, the appearance of the interior parts is deteriorated.
The recent process developments focus on improved process economy, including increased recycling of raw materials and simple process steps to separate undesired hydrocarbons components from the polymer.
It has been proposed to recover the polymer from the reactor by use of a separating device, such as a hydrocyclone. The hydrocyclone separates a part of the fluid phase from the obtained polymer, thereby producing a concentrated slurry. The separated fluid phase is directly recycled back into the polymerization reactor. While this is advantageous in terms of process economy, disadvantageously, any impurities contained in the fluid are also recycled back to the polymerization zone. This results in an accumulation of these impurities in the reaction mixture over time, and thereby the amount of impurities contained in the obtained polymer will increase. In turn, additional purification steps of the polymer become necessary, which is counterproductive for the overall process economy.
A simple method to remove unwanted hydrocarbons from the obtained solid polymer is to evaporate them. This is usually achieved by reducing the pressure so that at least part of the hydrocarbons is evaporated. Additionally, the temperature may be increased at the same time to facilitate the evaporation. However, any less volatile impurities eventually present in the polymerization mixture cannot be separated from the solid polymer therewith and remain in the product.
EP-A-1591459 relates to a process for producing olefin polymers, wherein the polymer slurry from a loop reactor is directed to a hydrocyclone, which divides the slurry flow into two streams. The overflow, which is rich in liquid, is directly recycled to the loop reactor without purification. The polymer, contained in the underflow, is further treated so as to obtain the final product.
EP-A-1118625 relates to a process for producing olefin polymers using two polymerization reactors connected in series. The reaction mixture, containing the polymer from the first reactor, is separated in a hydrocyclone, and the overflow recycled into the first reactor. The underflow, containing the polymer, is further reacted in the second reactor, followed by the separation in a second hydrocyclone. The overflow of the second hydrocyclone is recycled into the first reactor, while the final product is obtained from the underflow.
EP-A-1118626 discloses a process for producing olefin polymers using two reactors connected in series. The reaction mixture of the first reactor is separated in a first hydrocyclone, and the overflow recycled into the first reactor. The underflow, containing the polymer, is further reacted in a second reactor. The reaction mixture of the second reactor is then separated in a second hydrocyclone. The overflow is distilled and one fraction is recycled into the first reaction, while a second fraction is recycled in the second reactor. The final product is obtained from the underflow.
International Patent Application Publication No. WO 2004/007566 discloses a process for passing a polymerization effluent to a high pressure flash where most of the fluid components are flashed and wherein a slip stream comprising diluent and minor amounts of monomer is separated from the bulk of the flashed fluid components. The process of this application aims at minimizing compression and cooling of product fluids prior to recycling, enabling the recovery and the recycling of monomer free diluent which may be used for the preparation of catalyst mud. Furthermore, the flashing step employed in accordance with the teaching of International Patent Application Publication No. WO 2004/007566 as well as the corresponding process step disclosed in EP-A-1591459 only aims at removing diluent and (optionally) monomer, while leaving all other components, including catalyst residues and impurities in the polymer stream.
However, the processes of the prior art are disadvantageous since impurities contained in the overflow are transferred back into the reactor and accumulate in the reaction mixture and finally in the obtained product. Further, while the distillation of the complete overflow has the result of reducing the impurities to a minimum, improved process economy could be obtained if only a suitable fraction of the overhead were passed through distillation steps.
International Patent Application Publication No. WO 02/34795 discloses a solution polymerization process. Likewise, International Patent Application Publication No. WO 2006/009951 and EP 0184935 relate to solution polymerization processes. Solution polymerization processes are a specific kind of polymerization processes with particular aspects concerning process control and regulation, not comparable with slurry processes as discussed in the above-given prior art documents.
EP 184935 furthermore is concerned with a process for recovering polymer from a solution of the polymer in a solvent, without providing any guidance concerning process control regarding the reduction of impurity contents.
In view of the problems of the processes of the prior art, it is the object of the present invention to provide an improved process for the production of polyolefins having an extra low impurity content while still maintaining a good production economy.